Optical sensors are used in optical sensor devices, such as image sensors, ambient light sensors, proximity sensors, hue sensors, and UV sensors, to convert optical signals into electrical signals, allowing detection of optical signals or image capture. An optical sensor, generally, includes one or more sensor elements and one or more optical filters disposed over the one or more sensor elements.
For example, a color image sensor includes a plurality of color filters disposed in an array, i.e., a color filter array (CFA). The CFA includes different types of color filters having different color passbands, e.g., red, green, and blue (RGB) filters.
Conventionally, absorption filters formed using dyes are used as color filters. Unfortunately, such dye-based color filters have relatively broad color passbands, resulting in less brilliant colors. Alternatively, dichroic filters, i.e., interference filters, formed of stacked dielectric layers may be used as color filters. Such all-dielectric color filters have higher transmission levels and narrower color passbands, resulting in brighter and more brilliant colors. However, the color passbands of all-dielectric color filters undergo relatively large center-wavelength shifts with changes in incidence angle, resulting in undesirable shifts in color.
Furthermore, all-dielectric color filters, typically, include a large number of stacked dielectric layers and are relatively thick. Consequently, all-dielectric color filters are expensive and difficult to manufacture. In particular, all-dielectric color filters are difficult to etch chemically. Lift-off processes are, therefore, preferred for patterning. Examples of lift-off processes for patterning all-dielectric color filters in CFAs are disclosed in U.S. Pat. No. 5,120,622 to Hanrahan, issued on Jun. 9, 1992, in U.S. Pat. No. 5,711,889 to Buchsbaum, issued on Jan. 27, 1998, in U.S. Pat. No. 6,238,583 to Edlinger, et al., issued on May 29, 2001, in U.S. Pat. No. 6,638,668 to Buchsbaum, et al., issued on Oct. 28, 2003, and in U.S. Pat. No. 7,648,808 to Buchsbaum, et al., issued on Jan. 19, 2010, which are incorporated herein by reference. However, lift-off processes are, generally, limited to a filter spacing of about twice the filter height, which makes it difficult to achieve all-dielectric CFAs suitable for smaller color image sensors.
In addition to transmitting visible light in color passbands, both dye-based and all-dielectric color filters also transmit infrared (IR) light, which contributes to noise. Therefore, a color image sensor, typically, also includes an IR-blocking filter disposed over the CFA. IR-blocking filters are also used in other optical sensor devices operating in the visible spectral range. Conventionally, absorption filters formed of colored glass or dichroic filters formed of stacked dielectric layers are used as IR-blocking filters. Alternatively, induced transmission filters formed of stacked metal and dielectric layers may be used as IR-blocking filters. Examples of metal-dielectric IR-blocking filters are disclosed in U.S. Pat. No. 5,648,653 to Sakamoto, et al., issued on Jul. 15, 1997, and in U.S. Pat. No. 7,133,197 to Ockenfuss, et al., issued on Nov. 7, 2006, which are incorporated herein by reference.
To avoid the use of an IR-blocking filter, induced transmission filters formed of stacked metal and dielectric layers may be used as color filters. Metal-dielectric optical filters, such as metal-dielectric color filters, are inherently IR-blocking. Typically, metal-dielectric color filters have relatively narrow color passbands that do not shift significantly in wavelength with changes in incidence angle. Furthermore, metal-dielectric color filters are, generally, much thinner than all-dielectric color filters. Examples of metal-dielectric color filters are disclosed in U.S. Pat. No. 4,979,803 to McGuckin, et al., issued on Dec. 25, 1990, in U.S. Pat. No. 6,031,653 to Wang, issued on Feb. 29, 2000, in U.S. Patent Application No. 2009/0302407 to Gidon, et al., published on Dec. 10, 2009, in U.S. Patent Application No. 2011/0204463 to Grand, published on Aug. 25, 2011, and in U.S. Patent Application No. 2012/0085944 to Gidon, et al., published on Apr. 12, 2012, which are incorporated herein by reference.
Typically, the metal layers in metal-dielectric optical filters, such as metal-dielectric color filters, are silver or aluminum layers, which are environmentally unstable and which deteriorate when exposed to even small amounts of water or sulfur. Chemically etching the silver layers exposes the edges of the silver layers to the environment, allowing deterioration. Therefore, in most instances, metal-dielectric color filters in CFAs are patterned by adjusting the thicknesses of only the dielectric layers to select different color passbands for the metal-dielectric color filters. In other words, different types of metal-dielectric color filters having different color passbands are required to have the same number of silver layers as one another and the same thicknesses of the silver layers as one another. Unfortunately, these requirements severely limit the possible optical designs for the metal-dielectric color filters.
The present invention provides metal-dielectric optical filters that are not subject to these requirements, which are particularly suitable for use in image sensors and other sensor devices, such as ambient light sensors, proximity sensors, hue sensors, and UV sensors.